In wireless communication systems, designers are constantly forced to make compromises between competing performance parameters when designing a transmission path for a communication device, such as a mobile device or a base station. With reference to FIG. 1, a typical transmission path for a wireless communication device is illustrated. The transmission path generally includes control and modulation circuitry 10 for encoding data to be transmitted via a modulated RF input signal RFIN; power amplifier circuitry (PA) 12 for amplifying the modulated RF input signal RFIN; an impedance matching network 14, which will be described below in further detail; and an antenna 16 for transmitting a modulated and amplified RF output signal RFOUT to a remote device. In almost any transmission path design, designers are forced to make tradeoffs between the normally competing parameters of efficiency and linearity. To minimize power consumption and heat generation, maximizing efficiency is imperative. To maximize the quality of the transmitted signal, maintaining linearity is often imperative.
Unfortunately, the more accurate transmission path designs are generally less efficient. While linearity and efficiency are important in transmission path design, other parameters, such as the effective operating frequency range (i.e., bandwidth) and signal gain, are also important to the overall design and tend to compete with one another. Different designers may weigh these various parameters differently. For example, the efficiency and bandwidth of the transmission path may take precedence over linearity in a mobile device, while linearity may take precedence over efficiency and bandwidth in a base station. Further, one designer of a particular type of communication device may have different priorities for the various parameters than another designer based on the particular application or price point for the communication device.
Notably, the operating frequency, output power, and type of modulation scheme provided by a communication device all significantly impact performance. For example, power amplifier circuitry 12 that employs a given type of amplifier may be relatively efficient within a first bandwidth of operation or first output power range, but may be relatively inefficient in a second bandwidth of operation or second output power range. Similarly, the same power amplifier circuitry 12 may be relatively efficient at amplifying a signal that was modulated using one modulation scheme, but may be relatively inefficient at amplifying a signal that was modulated using a second modulation scheme. These conflicting design parameters are most problematic with communication devices that employ different modulation schemes, support different or wide bandwidths of operation, and need to operate over a wide range of output power levels. In essence, there are no amplifier designs that remain highly efficient at different power levels while operating over wide bandwidths and on signals that were modulated with different modulation schemes. As a result, designers are developing techniques to dynamically change the configuration of various aspects of the transmission path based on characteristics of the RF input signal RFIN in an effort to improve overall system performance.
One aspect of the transmission path that is being dynamically configured during operation is the actual impedance of the impedance matching network 14. As illustrated in FIG. 1, the impedance matching network 14 resides between the power amplifier circuitry 12 and the antenna 16 and is generally used to match the output impedance of the power amplifier circuitry 12 with the load impedance presented by the antenna 16. In theory, matching the output impedance of the power amplifier circuitry 12 with the load impedance presented by the antenna 16 results in maximum power transfer from the power amplifier circuitry 12 to the antenna 16. In practice however, such theoretical matching is at best approximated because the impedance matching network 14 also impacts many operating parameters of the power amplifier circuitry 12. For instance, the impedance of the effective load presented to the power amplifier circuitry 12 may greatly affect the linearity, output power, and efficiency of the power amplifier circuitry 12, and as such, the effective load presented to the power amplifier circuitry 12 may be more critical than perfectly matching the output impedance of the power amplifier circuitry 12 to the load impedance presented by the antenna 16.
As such, the impedance of the impedance matching network 14 may be dynamically adjusted during operation based on characteristics of the RF input signal RFIN, such as the center frequency, amplitude, the modulation of the RF input signal RFIN, and the desired output power. As illustrated in FIGS. 2 and 3, impedance control circuitry 18 may be added to the transmission path and used to dynamically adjust the variable impedance of the impedance matching network 14 based on characteristics of the RF input signal RFIN during operation and the desired output power. For the transmission path illustrated in FIG. 2, the impedance control circuitry 18 is configured to receive and analyze the RF input signal RFIN, and based on analyzed characteristics of the RF input signal RFIN and the desired output power, dynamically adjust the impedance of the impedance control circuitry 18 in a defined manner using an impedance control signal SZC. For example, the impedance control circuitry 18 could continuously adjust one or more variable impedance elements of the impedance matching network 14 based on the amplitude and center frequency of the RF input signal RFIN.
The transmission path is designed to support multiple channels where each channel is generally associated with a different center frequency. For each center frequency, or channel, the impedance control circuitry 18 may include a number of control values that correspond to the different possible amplitude values of the RF input signal RFIN and the different available output power levels. In operation, the impedance control circuitry 18 will continuously identify a control value based on the center frequency and amplitude of the RF input signal RFIN and the selected output power level and generate a corresponding impedance control signal SZC, which is used to adjust the impedance matching network 14 to provide the desired impedance for the given center frequency and amplitude of the RF input signal RFIN and the desired output power level.
For the transmission path illustrated in FIG. 3, the control and modulation circuitry 10 is configured to present to the impedance control circuitry 18 a parameter signal SP that provides information bearing on one or more characteristics of the RF input signal RFIN, which is concurrently being presented to the power amplifier circuitry 12 for amplification. Based on the parameter signal SP, the impedance control circuitry 18 will generate a corresponding impedance control signal SZC to adjust the impedance matching network 14 to provide the desired impedance for the given center frequency and amplitude of the RF input signal RFIN and the selected output power level. With the embodiment in FIG. 3, the impedance control circuitry 18 does not need to analyze the actual RF input signal RFIN, as provided the embodiment of FIG. 2. Instead, the impedance control circuitry 18 only needs to determine the appropriate impedance to select based on the parameter signal SP and provide an appropriate impedance control signal SZC to the impedance matching network 14.
Existing transmission path designs that employ modulated impedances that are presented to the power amplifier circuitry 12 focus on impedances at the center frequency of the RF input signal RFIN. The impedances at the various harmonics of the RF input signal RFIN have been ignored. As a highly simplified example, assume that the transmission path is used to transmit a first signal at a first center frequency f1C and a second signal at a second center frequency f2C. The first center frequency f1C and the second center frequency f2C are at different frequencies in different defined bandwidths of operation. Further assume that to meet desired performance specifications, designers have determined that the impedance matching network 14 should “ideally” provide a first impedance z1C when the RF input signal RFIN is provided at the first center frequency f1C and provide a second impedance z2C when the RF input signal RFIN is provided at the second center frequency f2C. In operation, the impedance control circuitry 18 will adjust the impedance matching network 14 to provide the first impedance z1C when the RF input signal RFIN is at the first center frequency f1C and provide the second impedance z2C when the RF input signal RFIN is at the second center frequency f2C. With reference to FIG. 4, the respective impedances z1C and z2C, which are provided at the first and second center frequencies f1C and f2C, are illustrated as impedance points (f1C and f2C) on a Smith chart.
As noted, existing load switching designs fail to take into consideration the impedances at the harmonics of the center frequencies of operation. However, applicants have discovered that the impedances at the harmonics of the center frequencies significantly impact the performance of the power amplifier circuitry 12 in particular and the transmission path in general. To meet a given performance specification, applicants have discovered that for any given operating condition, there are “ideal” impedances for the harmonics of the center frequencies in addition to an “ideal” impedance for the center frequency of an RF input signal RFIN. These “ideal” impedances for the given operating conditions may vary based on the desired performance specification for a particular design.
Continuing with the prior example, assume that designers determined that the impedance matching network 14 should “ideally” provide the first impedance z1C when the RF input signal RFIN is provided at the first center frequency f1C and provide the second impedance z2C when the RF input signal RFIN is provided at the second center frequency f2C. Again, the impedance control circuitry 18 will adjust the impedance matching network 14 to provide the first impedance z1C when the RF input signal RFIN is at the first center frequency f1C and provide the second impedance z2C when the RF input signal RFIN is at the second center frequency f2C. With reference to FIG. 5, the respective impedances z1C and z2C, which are provided at the first and second center frequencies f1C and f2C, are illustrated as impedance points (f1C and f2C) on a Smith chart.
If only the impedances at the first and second center frequencies f1C and f2C are considered when designing the impedance matching network 14, applicants have discovered that the actual impedance points for the harmonics associated with the first and second center frequencies f1C and f2C may differ significantly from what would be considered the “ideal” impedance points for the respective harmonics. The result is compromised performance. With continued reference to the Smith chart of FIG. 5, assume that the actual impedance points for the second and third harmonics f12H and f13H associated with the first center frequency f1C and the second and third harmonics f22H and f23H associated with the second center frequency f2C are not considered “ideal” and are provided as a result of only considering the impedances at the first and second center frequencies f1C and f2C of operation. As illustrated by circles on the Smith chart, exemplary “ideal” impedance points or ranges (f12H(IDEAL), f13H(IDEAL), f22H(IDEAL), and f23H(IDEAL)) for the second and third harmonics f12H and f13H associated with the first center frequency f1C and the second and third harmonics f22H and f23H of the second center frequency f2C are illustrated. As one can see, the differences between the actual impedances and the desired impedances for the different harmonics vary greatly.
While the impedances at the center frequencies of operation may be considered “ideal” for the given performance specification, the transmission path could perform significantly better if the impedance matching network 14 were configured and controlled to provide the “ideal” impedances at the center frequency and at least one of the harmonics associated with the center frequency for any given RF input signal RFIN, as illustrated in FIG. 6. In this example, for an RF input signal RFIN at the first center frequency f1C, the impedance matching network 14 would preferably provide “ideal” impedances at the first center frequency f1C as well as the second and third harmonics f12H(IDEAL) and f13H(IDEAL) associated with the first center frequency f1C. For an RF input signal RFIN at the second center frequency f1C, the impedance matching network 14 would preferably provide “ideal” impedances at the second center frequency f2C as well as the second and third harmonics f22H(IDEAL) and f23H(IDEAL) associated with the second center frequency f2C.
Accordingly, there is a need to dynamically control the impedance matching network of a transmission path to provide desired impedances at one or more of the harmonics associated with the center frequency of an RF input signal in addition to providing a desired impedance at the center frequency of the RF input signal.